A field-effect type transistor is widely used as a unit electron element of a semiconductor memory integrated circuit, a high-frequency signal amplification element, a liquid crystal driving element or the like. This field-effect type transistor is an electron device which is most practically used in recent years.
A thin film transistor (TFT) is one of field-effect type transistors. With a remarkable development of an image display apparatus in recent years, this thin film transistor (TFT) is widely used in various kinds of image display apparatuses as a switching element which applies a driving voltage to a display element to allow the image display apparatus to drive. Various kinds of an image display apparatus include a liquid crystal image display unit (LCD), an electroluminescence image display unit (EL) and a field emission display (FED).
As a material for a channel layer of a TFT, a silicon semiconductor compound is most widely used. Generally, a silicon single crystal is used in a high-frequency amplification element, an integrated circuit element or the like which needs a high-speed operation. In order to meet a demand for an increased area, amorphous silicon is used in a liquid crystal driving element or the like.
Moreover, since a high temperature of 800° C. or more is needed to attain crystallization, for example, it is difficult to form a crystalline silicon-based thin film on a glass substrate and an organic substance substrate. Therefore, a crystalline silicon-based thin film can be formed only on an expensive heat-resistant substrate formed of silicon wafer, quartz, or the like. Moreover, a large amount of energy and an increased number of steps are needed in manufacturing.
Furthermore, in the case of a TFT using a crystalline silicon-based thin film, it is normally restricted to have a top gate-type configuration. Therefore, cost reduction by reducing the number of masks or the like is usually difficult.
An amorphous silicon semiconductor (amorphous silicon) which can be formed at comparatively low temperatures has a slow switching speed as compared with a crystalline silicon semiconductor. Therefore, an amorphous silicon semiconductor may not be effective to display a high-speed animation when used as a switching element for driving an image display unit.
Furthermore, when irradiated with visible light, this semiconductor active layer exhibits conductivity and causes current leakage. In this case, there is a possibility that the semiconductor active layer may malfunction, and its characteristic as a switching element may deteriorate. Therefore, preparing a light-shielding layer which intercepts visible rays is known. For example, a metal thin film is used as a light-shielding layer.
If a light-shielding layer which is formed of a metal thin film is provided, the number of steps increases. Moreover, since this light-shielding layer has a floating potential, it is required that a light-shielding layer be formed at a ground level. In this case, a disadvantage occurs that a parasitic capacitance generates.
An element formed of a silicon-based semiconductor film constitutes the mainstream as a switching element for driving an image display apparatus. The reason therefor is that a silicon-based thin film is stable, is easy to process, has a high switching speed and exhibits various favorable performances. Generally, this silicon-based thin film is manufactured by the chemical deposition (CVD) method.
Moreover, as one of the structures of the conventional thin film transistor (TFT), an inverted-staggered structure can be given. This inverted-staggered structure is a structure in which a gate electrode, a gate-insulating layer, a semiconductor layer such as a hydrogenated amorphous silicon (a-Si:H) layer, a source electrode and a drain electrode are sequentially arranged on a substrate such as glass. This thin film transistor is used as a driving element for driving a flat panel display, or the like. A flat panel display is represented by an active matrix type liquid crystal display in the field of an image sensor and a device with a large area. However, in these applications, conventional thin film transistors using amorphous silicon are required to have a higher operation speed to meet the advancement in function.
Under such circumstances, in recent years, a transparent semiconductor thin film formed of a metal oxide has attracted attention as a material for a channel layer of a TFT.
This metal oxide excels a silicon-based semiconductor film in stability. Various technologies have been studied widely. As one of such technologies, a PLD (pulse laser deposition) method can be mentioned, in which an amorphous transparent semiconductor film composed of indium oxide, gallium oxide and zinc oxide is formed so as to allow a thin film transistor to drive.
Furthermore, an active matrix-type image display apparatus such as a LCD (liquid crystal display) and an organic EL (Electro Luminescence) display has been widely used in view of display performance, energy saving or the like. In particular, it has come to almost constitute the mainstream as displays of cellular phones, PDAs (Personal Digital Assistant) and personal computers, laptop computers and TVs. Generally, a TFT (field-effect type thin film transistor) substrate is used in these displays.
For example, a liquid crystal display has a configuration in which a display material such as liquid crystal is filled between a TFT substrate and an opposing substrate, and a voltage is selectively applied to this display material for each pixel. Here, a TFT substrate means a substrate in which a TFT using a semiconductor thin film (also referred to as a semiconductor film) such as an amorphous silicon thin film or a polycrystalline silicon thin film is arranged. The above-mentioned image display apparatus is driven by the active matrix circuit of a TFT. Since a TFT is arranged in the shape of an array, a TFT substrate is also referred to as a “TFT array substrate”.
Meanwhile, in a TFT substrate used for a liquid crystal display or the like, a set of a TFT and one pixel of the screen of a liquid crystal display (this set is referred to as one unit) are arranged longitudinally and laterally on a glass substrate. In a TFT substrate, for example, a gate wire is arranged longitudinally at equal intervals on a glass substrate and a source wire and a drain wire are arranged laterally at equal intervals. In addition, a gate electrode, a source electrode, and a drain electrode are respectively formed in the above-mentioned unit which constitutes each pixel.
A transistor using the above-mentioned silicon thin film is produced by using a silane-based gas, and hence, it is disadvantageous in respect of safety or equipment cost. In addition, an amorphous silicon thin film has an electron mobility which is as low as about 0.5 cm2/Vs when used in a TFT. In addition, since an amorphous silicon thin film has a small band gap, it may absorb visible rays to cause malfunction. Moreover, a polycrystalline silicon thin film requires a heating process which is conducted at relatively high temperatures, needs a large amount of energy cost, and it is hard to be formed directly on a large-sized glass substrate.
Under such circumstances, a TFT using an oxide semiconductor thin film in which a film can be formed at low temperatures has been actively developed. In addition, a semiconductor device using an oxide semiconductor thin film or the like has also been developed with the development of the above-mentioned TFT.
For example, Patent Document 1 discloses a technology of a thin film transistor in which a gate-insulating film including a first insulating film which has high insulating performance and a second insulating film which is formed of an oxide (for example, SiO2) is formed on a gate electrode and a semiconductor layer using ZnO or the like is formed on the second insulating film. This thin film transistor has a low leak current level in the OFF region, a high mobility and excellent switching characteristics.
In addition, Patent Document 2 discloses a technology of a semiconductor device in which a semiconductor layer is formed of ZnO (zinc oxide) and aluminum is used as a material of an electrode and wiring, which device is free from defects and troubles caused by electric erosion.
Furthermore, Patent Document 3 discloses a technology of a transparent conductive film formed of an amorphous oxide which contains indium (In) and zinc (Zn) as a main cation element and has an atomic ratio of In (In/(In+Zn)) within a range of 0.8 to 0.9 is provided on a transparent substrate, and a method of producing the same. According to this technology, the transparent conductive film has conductivity and light transmittance which are practically satisfactory, and is excellent in thermal stability such as etching characteristics and specific resistance. Furthermore, this transparent conductive film can be provided on a substrate at lower substrate temperatures.
Moreover, Patent Document 4 discloses a technology of a semiconductor device which uses a transparent oxide film. This semiconductor device is provided with a P-type region and an N-type region, and uses in the N-type region an amorphous oxide having an electron carrier concentration of less than 1018/cm3 or an amorphous oxide having a tendency that the electron mobility increases with an increase in electron carrier concentration.
Furthermore, Patent Document 5 discloses a technology of an active matrix-type image display apparatus. This image display apparatus has an amorphous oxide having an electron carrier concentration of less than 1018/cm3 as an active layer of a field-effect type transistor.    Patent Document 1: JP-A-2003-86808    Patent Document 2: JP-A-2004-273614    Patent Document 3: JP-A-7-235219    Patent Document 4: JP-A-2006-165532    Patent Document 5: JP-A-2006-165528